Not starting an argument with anyone, just answering
@Zblurth questions.
Originally, the Japanese did not find sex or sexual acts indecent or disgusting at all. Sure the Japanese people of old did have the concepts of purity and relationship similar to the modern age, but sex itself was not something dirty to them. Even when the Portuguese came, they weren't really affected by the concepts of monogamy as the Portuguese's movements were restricted and the general population continued as they were.
Enter the westerners, or specifically, the Americans, who enforced the idea that sex and prostitution are depraved and disgusting. This ideals were prevalent in Meiji period, but the effect was low. While the Japanese understood why the Americans consider it so, they in general couldn't care less, and literature that mocked American fetish such as pedophilia, prostitution, nudity and orgies were rampant during the Meiji era. It wasn't outlawed, and was seen as a kind of satire of how restricted American society was.
Then the Japanese lost the world war, and among others, they were forced to accept the American values regarding sex and nudity. For decades, this value were fought by the general public and the rules against it were generally ignored. Then the Japanese government back then decided to accept minimal compliance by letting them have their sex and nudity with some restriction. The obscenity act is still there, but they redefined what is obscene.
The baseline:
1) Genital (penis & vagina) depictions are no-no. It has to either not be drawn or censored even if drawn. As long as the genitals aren't shown, it does not violate the obscenity act.
If you want to sell in bookstores:
1) Same as the baseline but for a bookstore exceeding a certain size, an adult-only section must be allocated for such literature.
If you want to sell in convenience stores:
1) Same as the baseline and any literature containing sex between blood-related family members is not allowed. That's why you see a lot of inseki and not incest these days.
If you want to film a public porn:
1) You need to have a permit to do it for the site you want to do it at.